The invention relates to a device for the protection of a superconducting magnetic coil assembly that is to be cryogenically cooled by a liquid coolant when there is an unintended transition from the superconducting state to the normal conducting state. In the device the magentic coil assembly is subdivided into several partial winding sections that are connected one after another; each partial winding section is connected in parallel with a protective resistor with a predetermined ohmic value such that the ends of adjacent partial winding sections that face one another are connected to the corresponding ends of their respective protective resistors by a common section of an electrical conductor. A protection device of this type for a superconducting magnetic coil assembly is disclosed in published German patent application DE-OS No. 23 01 152.
Superconducting magnetic coils, such as those of a typical coil assembly, can frequently also be operated with short-circuiting. This is because once the magnetic field of such a coil has been set up there is virtually no need to feed in additional electrical energy from outside to maintain this field. To store the electrical energy that has been fed into the coil, it is therefore possible to short-circuit its winding at its ends by means of a persistent current switch with the lowest possible resistance (cf. published German patent application DE-OS No. 25 21 328, German patent DE-PS No. 23 24 371). The current then flows almost undamped in the resulting short-circuited electric circuit, and all the electrically conducting connections between the low-temperature area of the coil and the external room temperature, such as the leads from the power supply that are necessary to excite the magnetic coil, can then be interrupted to reduce the heat flow into the low-temperature area of the coil.
In the larger superconducting magnetic coils substantial quantities of energy, which might be in the MJ range, can be stored. These magnetic coils in particular are severely endangered in the event of an unintended transition from the superconducting operating state to the normal-conducting state even if this transition (also known as a "quench") initially occurs only in part of the coil. Due to the low heat capacity of the superconducting coil wires, the wire very rapidly reaches a high temperature after the transition from the superconducting to the normal conducting state, because of the resulting increase in resistance. At the same time its specific resistance also rises very rapidly, causing the heating to accelerate further. The result in this case is over-voltages, which put stress on the insulation. In order to protect the larger superconducting magnetic coils from damage or destruction caused by overheating or by electrical arc-overs, special procedures are required. These procedures include, for example, the division of the magnetic coils into several partial winding sections, which are bridged in each case with ohmic protective resistors (cf. published German patent application DE-OS No. 23 01 152), semi-conductor diodes (cf. published German patent application DE-OS No. 16 14 964) or arresters or overvoltage diverters (cf. published German patent application DE-OS No. 17 64 369), in order to limit the voltage. Another typical technique to limit the temperature in the superconducting magnetic coil is to accelerate the proliferation of the normally conducting areas. This can be accomplished with a current impulse through the coil winding or with a common winding form, made for example of aluminum, which is heated by eddy currents and generates new normally conducting areas in the magnetic coil (cf. "IEEE Transactions on Magnetics", Vol. MAG-15, No. 1, January 1979, pages 855 to 859). In addition, it is also possible to promote the quench proliferation by means of electrical heating elements on the windings, which are activated by a special quench detector and fed by an external power supply (cf. "IEEE Transactions on Magnetics," Vol. MAG-17, No. 5, September 1981, pages 1815 to 1822).
For purposes of reliability, however, it is frequently desirable to provide a passive protection device for a magnetic coil of this type, which performs its voltage-limiting and temperature-limiting functions without the activation of active elements, such as quench detectors, switches and externally-fed heaters.
The parallel protective resistors at the partial winding sections of the superconducting magnetic coil, which are provided for the protection device according to previously discussed published German patent application DE-OS No. 23 01 152, are to be regarded as a protection device of this kind. The protective effect of these resistors, which is to form a parallel path to the quenched, and therefore high-impedance winding for the current flowing in the other windings, raises a question as to the thermal loading capacity of these resistances. In the larger magnetic coils an output of up to 100 kW and more might have to be converted in such a resistance as the result of a quench. If the quench does not extend to the other partial winding sections, then the entire stored energy of the magnetic coil would be converted into heat in this resistor. In some applications it is desirable to arrange the resistances in the cryogenic environment close to the coil in order to avoid heat leakage through the many electrical connections between the partial winding sections and the protection resistors. In this case effective removal of thermal power may not be possible. In other words, the greater part of the energy would have to be absorbed by the resistor by raising the temperature of its mass. In order to limit the heating to a predetermined maximum temperature of, for example, a few 100.degree. K., the mass of each resistor would have to be relatively large (of the order of several tens of kilograms). Protective resistors having such a large mass, however, are often undesirable.